1. Field of the Invention
The present invention relates to a method of joining a member to a diecast article wrapping thereabout in diecasting.
2. Description of the Prior Art
In order to produce a single part made of two different metals, it has been well known to arrange a previously formed metal member in a mold into which a second metal is then poured so as to surround the first metal, thereby rigidly joining the two metals. Frequently, such a method has also been used in diecasting, in making precision castings. In this case, with conventional diecasting machines it is necessary to form an aperture in dies for inserting a member to be joined or to form a boss on the dies for supporting the member to be joined. When joining one end of an elongated member such as a shaft to another surrounding metal, however, dies are needed to form an aperture extending therethrough and to provide any type of positioning fastening means for rigidly supporting the member relative to the dies to prevent the member from being moved by the flowing movement of the molten metal at a high speed, so that the dies become unavoidably very complicated in construction.
In a conventional method of joining a metal to another metal cast thereabout with a diecasting machine, because of the solid metal which has been previously arranged in dies, the molten metal in contact with the solid metal solidifies for a short period of time. More important still, because the molten metal forced into the dies has already lowered its temperature, the joint between the solid metal and the surrounding cast metal is not necessarily satisfactory, and in most cases a metallurgically bonded joint between the two metals cannot be obtained.
In order to solve these problems, it has been suggested that a member 1 to be joined is previously formed with notches 1a to obtain a mechanical joint with a surrounding metal 2 as shown in FIG. 1. In this case, however, the member 1 to be joined must be formed with notches at an extra cost.
In the case of heat-resistant impeller with a shaft such as a turbocharger rotor as shown in FIGS. 2a and 2b, the impeller 3 is made of a heat-resistant alloy such as nickel, cobalt or iron base alloy by a precision casting of the lost wax process and thereafter the shaft made of carbon steel or low alloy steel is joined to the impeller by friction welding, electron-beam welding or the like. In general, however, such joining methods are very difficult because of their severe welding conditions which should be strictly controlled. If the welding conditions are not precisely controlled, the welded portions are prone to develop cracks in the welds resulting in the high percentages of rejected articles. With parts are used at high rotating speeds and high temperatures as is the turbocharger rotor, rigid joints sufficient to resist such severe use conditions are of course needed. In the usual lost wax process, however, it is difficult to join metal members by casting one to surround the other. In addition, with the conventional joining method of casting one to surround the other, a satisfactory joint cannot be obtained as above described.